The Space Division Multiple Access (SDMA) scheme achieves multiple access which can avoid mutual interference by separating users spatially based on the beam pattern formed by the adaptive array antenna (AAA) provided at the base station while allowing a plurality of users to share the same frequency and the same time slot. In order to appropriately form the beam pattern that avoids mutual interference according to the latest radio propagation condition of each terminal, this scheme uses reversibility of the propagation path of the communication system which employs the Time Division Duplex (TDD) scheme. In other words, the base station calculates the weight of AAA based on an uplink signal received from each terminal and corrects it if necessary, and then multiplies a downlink signal by this weight and transmits. Thus the weight multiplied to the downlink signal is calculated based on the uplink signal. Here, if the terminal moves at a high speed, when the base station transmits a downlink signal, the terminal may be away from the position where it had transmitted the uplink signal which the base station used to calculate the weight. In this case, the effect of preventing the interference by the beam pattern formed by the AAA fades and the received signal quality at each terminal deteriorates, resulting in decline in the throughput of the system.
In order to deal with the above mentioned problems, the conventional art suggests a channel assignment scheme in which the interference generated by movement of a terminal is avoided and the throughput characteristic is improved by assigning basic channels and spatial channels in order of moving speed of each terminal in consideration of the moving speed of each terminal (See Patent Document 1).
However, according to Patent Document 1, it is necessary to detect moving speed (that is, fading speed) of each terminal. Although some detection methods of moving speed have been suggested, most of them need to be further verified in terms of effectiveness, usefulness, accuracy or the like, and thus they are inconvenient. In addition, there is a method that uses highly effective and accurate GPS (Global Positioning System). However, the GPS is disadvantageous in that the cost is high and fading speed cannot be detected.
Further, even when channels are assigned in consideration of only terminal movement, for example, if a spatial channel is assigned to a high-priority terminal (e.g. a terminal performing VoIP: Voice over Internet Protocol) that requires QoS (Quality of Service), the terminal becomes more likely to be interfered by another spatial channel on the same frequency. Therefore, appropriate service cannot be provided even through its priority is high. It should be noted that, in this specification, “basic channel” refers to available carrier (frequency) and time slot and “spatial channel” refers to a channel which uses the same frequency and time slot as those of the basic channel but for which spatial multiplexing is performed based on the beam pattern of antenna formed by the AAA at the base station. Further, it should be noted that a basic channel can be a spatial channel when spatial multiplexing is performed, however, for convenience of explanation, such channel is also referred to as “basic channel”.    Patent Document 1: Japanese Patent Application Laid-Open No. 9-215052